Simultaneous Weather FX

[OS]

Spoiler: Game_Screen

#==============================================================================
# ** Game_Screen with Multiple Weather Support
# ** Edit by OS
#------------------------------------------------------------------------------
#  This class handles screen maintenance data, such as change in color tone,
#  flashing, etc. Refer to "$game_screen" for the instance of this class.
#==============================================================================

class Game_Screen
  #--------------------------------------------------------------------------
  # * Public Instance Variables
  #--------------------------------------------------------------------------
  attr_reader   :tone                     # color tone
  attr_reader   :flash_color              # flash color
  attr_reader   :shake                    # shake positioning
  attr_reader   :pictures                 # pictures
  attr_reader   :weathers                 # weather types and respective powers
  #--------------------------------------------------------------------------
  # * Object Initialization
  #--------------------------------------------------------------------------
  def initialize
    @tone = Tone.new(0, 0, 0, 0)
    @tone_target = Tone.new(0, 0, 0, 0)
    @tone_duration = 0
    @flash_color = Color.new(0, 0, 0, 0)
    @flash_duration = 0
    @shake_power = 0
    @shake_speed = 0
    @shake_duration = 0
    @shake_direction = 1
    @shake = 0
    @pictures = [nil]
    for i in 1..100
      @pictures.push(Game_Picture.new(i))
    end
    #Weathers = [[weather_type,weather_max],[etc.]]
    @weathers = [[0,0.0],[0,0.0]]
    @durations = [0,0]
    @weather_type_target = 0
    @weather_max_target = 0.0
    @weather_duration = 0
  end
  #--------------------------------------------------------------------------
  # * Start Changing Color Tone
  #     tone : color tone
  #     duration : time
  #--------------------------------------------------------------------------
  def start_tone_change(tone, duration)
    @tone_target = tone.clone
    @tone_duration = duration
    if @tone_duration == 0
      @tone = @tone_target.clone
    end
  end
  #--------------------------------------------------------------------------
  # * Start Flashing
  #     color : color
  #     duration : time
  #--------------------------------------------------------------------------
  def start_flash(color, duration)
    @flash_color = color.clone
    @flash_duration = duration
  end
  #--------------------------------------------------------------------------
  # * Start Shaking
  #     power : strength
  #     speed : speed
  #     duration : time
  #--------------------------------------------------------------------------
  def start_shake(power, speed, duration)
    @shake_power = power
    @shake_speed = speed
    @shake_duration = duration
  end
  #--------------------------------------------------------------------------
  # * Set Weather
  #     type : type
  #     power : strength
  #     duration : time
  #--------------------------------------------------------------------------
  def weather(type, power, duration=20, id=-1)
    @weather_type_target = type
    if @weather_type_target != 0
      index = id == -1 ? empty_index : id
      @weathers[index][0] = @weather_type_target
    end
    if @weather_type_target == 0
      @weather_max_target = 0.0
    else
      @weather_max_target = (power + 1) * 4.0
    end
    @durations[index] = duration
    if @durations[index] == 0
      @weathers[index][0] = @weather_type_target
      @weathers[index][1] = @weather_max_target
    end
  end
  #--------------------------------------------------------------------------
  # * Empty Index
  #--------------------------------------------------------------------------
  def empty_index
    for i in 0..@weathers.size - 1
      if @weathers[i][0] == 0
        return i
      end
    end
    return 0
  end
  #--------------------------------------------------------------------------
  # * Frame Update
  #--------------------------------------------------------------------------
  def update
    if @tone_duration >= 1
      d = @tone_duration
      @tone.red = (@tone.red * (d - 1) + @tone_target.red) / d
      @tone.green = (@tone.green * (d - 1) + @tone_target.green) / d
      @tone.blue = (@tone.blue * (d - 1) + @tone_target.blue) / d
      @tone.gray = (@tone.gray * (d - 1) + @tone_target.gray) / d
      @tone_duration -= 1
    end
    if @flash_duration >= 1
      d = @flash_duration
      @flash_color.alpha = @flash_color.alpha * (d - 1) / d
      @flash_duration -= 1
    end
    if @shake_duration >= 1 or @shake != 0
      delta = (@shake_power * @shake_speed * @shake_direction) / 10.0
      if @shake_duration <= 1 and @shake * (@shake + delta) < 0
        @shake = 0
      else
        @shake += delta
      end
      if @shake > @shake_power * 2
        @shake_direction = -1
      end
      if @shake < - @shake_power * 2
        @shake_direction = 1
      end
      if @shake_duration >= 1
        @shake_duration -= 1
      end
    end
    for i in 0..@weathers.size - 1
      if @durations[i] >= 1
        d = @durations[i]
        @weathers[i][1] = (@weathers[i][1] * (d - 1) + @weather_max_target) / d
        @durations[i] -= 1
        if @durations[i] == 0
          @weathers[i][0] = @weather_type_target
        end
      end
    end
    if $game_temp.in_battle
      for i in 51..100
        @pictures[i].update
      end
    else
      for i in 1..50
        @pictures[i].update
      end
    end
  end
end

This is an edit I made of Game_Screen that is supposed to allow two simultaneously active weather effects. Unfortunately, it keeps giving me an error at line 92 (no implicit conversion from nil to Integer)

Any ideas?